Add a StackingContext in preparation for z-index drawing.
Add a StackingContext in preparation for z-index drawing.

Kozea/WeasyPrint,Kozea/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,marclaporte/WeasyPrint,andrewleech/WeasyPrint,marclaporte/WeasyPrint,andrewleech/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint